dewar wiring

[Paul Jorden]

Posted to CCD-world:
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Andre, & CCD-world,

> We are replacing the constantan wiring in a dewar by the copper 
> wiring. Wondering whether there is any reason to use something else 
> than ordinary insulated multi-strand wire ? 
> 
We've gone through various methods over the years-
copper, stainless-steel, beryllium-copper, manganin, etc.

Here are some comments-
	Copper obviously solders easily, and has the best electrical 
conductivity, and highest thermal conductivity.  A thin copper wire is probably 
ideal, but fragile.  Hence the flex-circuit idea (as recently suggested) is 
attractive, because it allows thin wire, and is robust, and I think good in 
vacuum.  If you need to build for various chip types, then you can use a 
'generic' flex-cable to go from the dewar connector to the CCD location.  We 
have used a pcb (with connector) at this point; thus, we would make wiring 
changes for different chips by making a new pcb.  Also wiring changes can fairly 
easily be made external to the cryostat.

Stainless steel is not worth it for our applications I think- hardest to solder.

Beryllium-copper has a good electrical/ thermal conductivity ratio.  It can be 
soldered, but is a bit springy, and joints are not wonderful.  Not recommended 
now.

I selected manganin as a good wire. It also has good e/t ratio, and can be 
soldered, although not as securely as copper.  I used to buy enammeled wire. It 
has the advantage over a much lower thermo-electric potential than constantin 
(with respect to copper).  I had worried about this potential for spurios 
'noise'.

In all cases any 'standard' plastic insulation sleeving is not ideal, and ptfe 
sleeved wires are recommended for vacuum use.  Certainly for multiple use, I 
suspect the flex-circuit wiring is a good idea.

Hope this helps. Regards to all,
Paul
-------------------------------------------------------------------
Paul Jorden:  prj at ast.cam.ac.uk        Web: http://www.ast.cam.ac.uk/RGO
Royal Greenwich Observatory, Madingley Road, Cambridge, CB3 0EZ, UK.
Phone: +44 (0) 1223- 374000  \direct phone- 374811 \RGO Fax- 374700




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